why insertBefore not appendChild?

        /**         * Evalulates a script in a global context.         */        globalEval: function(data) {            if (data && REG_NOT_WHITE.test(data)) {                // Inspired by code by Andrea Giammarchi                // http://webreflection.blogspot.com/2007/08/global-scope-evaluation-and-dom.html                var head = doc.getElementsByTagName('head')[0] || doc.documentElement,                    script = doc.createElement('script');                // It works! All browsers support!                script.text = data;                // Use insertBefore instead of appendChild to circumvent an IE6 bug.                // This arises when a base node is used.                head.insertBefore(script, head.firstChild);                head.removeChild(script);            }        }    });

经常看大家使用insertBefore，而不用appendChild
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LC(============================) 12:23:42
两个BUG
1。 head没加载完毕 会报未运行是异常
2。 如果head部分有base标签 会引发异常